Why don't we coat the pressure vessels of nuclear reactors in boron to reduce neutron embrittlement? Neutron embrittlement is a big problem in reactor design, as high energy neutrons cause lattice defects in materials like steel by slamming into iron nuclei. This limits the lifetime of the reactor, because nobody likes a pressure vessel failure.
In order to combat this, why don't we coat the inside of a pressure vessel in boron? The boron would absorb the neutrons and thus the steel would avoid embrittlement. 
 A: While you can do that, you can also just take a thicker piece of steel as it's typically one of the cheapest materials you can get. Energy spectrum is typically not that hard as neutrons are usually moderated (if it's not fast-neutron reactor). 
But embrittlement in pressure vessel is not a major concern during reactor design. Neutron economy is the most important as it has direct impact on nuclear reactor economy. Every neutron = $ (you can ether trade it for fuel regeneration or lower fuel enrichment = lower costs).
In the nuclear reactor design it is preferred to use as much neutrons as possible, and hence try to reflect neutrons back to the core rather than dump them into the pressure vessel. Without reflector you will have very high non-uniformity of neutron flux in the volume of the core and it will be hard to operate. 
In VVER PWR reactors reflector is made from water-steel sandwich.
In graphite-based reactors reflector is also made from graphite.
You can see reactor composition example here: https://www.researchgate.net/figure/Horizontal-Top-and-vertical-Bottom-cross-section-of-3D-60-0-symmetric-two-energy_fig8_309576283

A: Fast neutrons are responsible of neutron embrittlement in the pressure vessel , so boron is not the best material to acts on fast neutrons .
In the new PWR EPR french reactor , a multi sections (12) heavy reflector  in stainless steel is fitted between the polygonal core and the cylindrical core barrel .
This reflector reduces the number of neutrons escaping from the core , flattens the power distribution and reduces the exposure of the vessel to fast neutron fluxes slowing down their energy .
